1. Field of the Invention
The present invention relates to a braking device for a vehicle, and particularly to a braking device of a brake-by-wire type using an electronic control.
2. Description of the Related Art
Conventionally, for such a braking device for a vehicle, there has been known a technique of Japanese Patent No. 4088802B. The braking device of Japanese Patent No. 4088802B includes: a motor rotatable in forward and reverse directions; a ball screw linearly-moving mechanism configured to convert a rotation of the nut interlocking with the motor into a linear motion in an axial direction of a bolt whose rotation is restricted; a brake operator linked with an rear end of the bolt so as to push the bolt; and a master cylinder having a master piston whose front end is positioned in a hydraulic chamber to which a wheel brake is connected, and whose rear end is linked with a front end of the bolt. In addition, the nut is rotatable and reciprocative in an axial direction whose retracting limit is set by a housing, and a stroke simulator is provided which is configured to shorten a relative distance between an input transferring member connected to the brake operator and the nut, in accordance with an input from the brake operator. When the nut advances, an elastic member in the stroke simulator advances together with the nut, and a thrust force of the input transferring member becomes transmittable to the bolt, to thereby regulate a stroke and input of the brake operator which would otherwise be consumed by the stroke simulator.
In the braking device of Japanese Patent No. 4088802B, because of the configuration described above, when the motor is actuated, a braking device functions as that of a brake-by-wire type, and when the motor is not actuated due to a failure of the motor or the like, a thrust force of the brake operator can be directly transmitted to the master piston.
However in the braking device of Japanese Patent No. 4088802B, when the motor is actuated, the nut is pressed to the retracting limited by a rotational force of the motor and a force exceeds a pushing force by the input transferring member. Therefore, in the case of a regenerative control or the like, when a torque of the motor is reduced, a force to press the nut to the retracting limit becomes weaker, and a problem may arise that the pushing force by the input transferring member is transmitted to the master piston. In this case, a transmitting force to the master piston cannot be suppressed up to an equivalent level to the pushing force by the input transferring member, and thus a satisfactory regenerative control cannot be performed.
In addition, in a case of a brake operation of an emergency or the like, a time lag may be disadvantageously occurred between the depression of the brake pedal and the actuation of the motor. In this case, after zeroing a gap with a rear end portion of the bolt, the input transferring member directly pushes the bolt until the motor is actuated, to thereby generate a pressure in the master cylinder. Afterward, when the motor is actuated, the nut is pressed to the retracting limit, and as the bolt and the input transferring member move away from each other, a reaction force on the brake pedal is switched from that of the master cylinder to that of the stroke simulator. Therefore, at the time point where the motor is actuated, a magnitude of a reaction force to the brake pedal becomes discontinuous, which brings an uncomfortable feeling to the driver.
Therefore, it would be desirable to provide a braking device as a brake-by-wire type by an electronic control in which even when the electronic control becomes incapable, a braking force can be accurately generated by the operation of the brake pedal.